Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/platform/graphics/ImageSource.cpp

Index: Source/platform/graphics/ImageSource.cpp
diff --git a/Source/platform/graphics/ImageSource.cpp b/Source/platform/graphics/ImageSource.cpp
index dee22c391543ab86059b18aaeefc05cede16b4c6..03ae1da016c7d082958c11d66d3d230bd2ea9b79 100644
--- a/Source/platform/graphics/ImageSource.cpp
+++ b/Source/platform/graphics/ImageSource.cpp
@@ -113,16 +113,16 @@ size_t ImageSource::frameCount() const
 PassRefPtr<NativeImageSkia> ImageSource::createFrameAtIndex(size_t index)
 {
     if (!m_decoder)
-        return 0;
+        return nullptr;
 
     ImageFrame* buffer = m_decoder->frameBufferAtIndex(index);
     if (!buffer || buffer->status() == ImageFrame::FrameEmpty)
-        return 0;
+        return nullptr;
 
     // Zero-height images can cause problems for some ports.  If we have an
     // empty image dimension, just bail.
     if (size().isEmpty())
-        return 0;
+        return nullptr;
 
     // Return the buffer contents as a native image.  For some ports, the data
     // is already in a native container, and this just increments its refcount.